Conventionally, there has been known an opening/closing device used in a flow passage such as a sewer (refer to Patent Document 1, for example). The opening/closing device is used to clean a section where garbage tends to be accumulated such as a slack portion or an inverted siphon structure of a flow passage. Such an opening/closing device usually dams the flow passage while a valve element (or a gate) is closed. Therefore, flowing sewage is gradually accumulated, and when a water level of the flow passage becomes equal to or more than a predetermined water level, the valve element falls down, and enters an open state, the water flows at once toward downstream of the flow passage, and the garbage can be flushed away even in the section that tends to accumulate the garbage. In other words, the flow passage can be cleaned.
It should be noted that in the opening/closing device described in PATENT DOCUMENT 1, two floats arranged in an up/down direction are used to open a valve element quickly. It should be noted that the lower float out of the two floats is thicker in the up/down direction than the upper float.